Internal combustion engines such as, but not limited to, diesel and gasoline engines, may include one or more temperature sensors at least partially disposed within the exhaust gas system. These temperature sensors may sense the temperature of the exhaust gas and may be used, at least in part, by an engine control system to adjust one or more properties of the engine such as, but not limited to, air/fuel ratio, boost pressure, exhaust regeneration cycles/duration, timing or the like. Because of the operating environment, the temperature sensors may be exposed to relatively harsh conditions including, but not limited to, vibration, exposure to debris, moisture and corrosive chemicals, large temperature ranges and rates of temperature change and relatively high continuous use operating temperatures. The conditions may degrade the performance of the temperature sensors and may, ultimately, render the temperature sensors unsuitable for their intended purpose.
Thin film resistive temperature detectors are a variety of temperature sensor used for detecting temperature in many applications, including but not limited to effluent or emissions from an engine. For example, such detectors may be used for detecting the exhaust gas temperature of an internal combustion engine. The exhaust gas temperature sensor may be part of an engine management system. Various operating parameters, such as fuel delivery, etc., may be adjusted based in part on a measured exhaust gas temperature.
Platinum metal film resistive temperature detectors are one particular variety of temperature sensor used for detecting effluent temperature. The platinum metal resistive element used in such temperature detectors is sensitive to environmental conditions. For example, a reducing atmosphere may cause migration of the platinum film of the resistive element from its substrate if oxygen in the surrounding atmosphere is below a threshold concentration. Significant loss of platinum from the resistive temperature detector resulting from decomposition or migration of the platinum may adversely affect the performance and life of the temperature detector.
Oxide/ceramic based resistive temperature sensing elements commonly referred to as negative temperature coefficient (NTC) elements and/or positive temperature coefficient (PTC) elements are also a particular variety of elements incorporated into a temperature sensor used for detecting effluent temperature. A NTC element generally refers to materials that experience a decrease in electrical resistance with increasing temperature, typically in a defined temperature range. A PTC element generally refers to materials that experience an increase in electrical resistance with increasing temperature. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase. Similar to platinum metal resistive elements, NTC/PTC elements used in such temperature sensors are also sensitive to environmental conditions, such as those discussed above, which may adversely affect the performance and life of the temperature detector.
The interior surfaces of a closed, or encapsulated, temperature sensor may react with the trapped oxygen in the closed environment, thereby reducing the oxygen concentration and leaving the platinum resistive element susceptible to damage from the resulting reducing environment. The volume of air which may be contained within the closed temperature sensor may be limited because too great an internal volume may insulate the resistive temperature detector element from the exterior of the sensor, greatly increasing the thermal time constant and reducing the performance of the sensor. Because of the restrictions on the internal volume of the enclosure, even if the interior surfaces of the enclosure have been pre-oxidized prior to final assembly of the sensor, further oxidation of the interior surfaces and/or contaminates which may occur over time may still reduce the oxygen concentration leaving the resistive element, such as platinum and/or PTC/NTC elements, susceptible to damage.
Open temperature probes, which do not provide a closed environment, are open to the outside atmosphere to allow oxygen exchange with the platinum film of the temperature detector in order to prevent the loss or migration of the metal film in the presence of a reducing atmosphere. While the open design may allow communication with the external atmosphere, the external atmosphere may not, necessarily, provide a sufficient oxygen concentration to prevent the loss of, or damage to, the thin film resistive element. Additionally, the open design may allow the entrance of contaminants, inhibit or otherwise negatively affect the substrate, the platinum film, the thermal response time, etc., of the temperature detector. Open probes may also be subject to the introduction of moisture, which, when combined with freezing temperatures and less than optimal mounting orientations and/or positions, can result in structural damage as the result of freeze/thaw forces applied thereto.